A particle beam therapy system is a medical apparatus for performing treatment by irradiating a charged particle beam onto a diseased site such as a cancer. The function, to be required, of a particle beam therapy system is to form an irradiation field in such a way as to provide a diseased site such as a cancer with a dose required for treatment and to provide other normal tissues with as few dose as possible. Irradiation field forming methods include the broad beam irradiation method and the scanning irradiation method.
In the broad beam irradiation method, at first, an irradiation field is enlarged by use of a scatterer or the like and then is formed to conform to a diseased site by use of a collimator, a bolus, or the like. Because its safety had been assured through clinical research, the broad beam irradiation method has most widely been adopted in a conventional particle beam therapy system. However, because the shape of a diseased site differs depending on a patient, or because even in a single and the same patient, a diseased site shrinks as a treatment proceeds, it is required to create a bolus each time the diseased site shrinks; therefore, there has been desired a more flexible irradiation field forming method. Accordingly, in recent years, there have actively been conducted R&Ds in which the scanning irradiation method is adopted in a particle beam therapy system.
The scanning irradiation method denotes a method in which a pencil-shaped thin charged particle beam is irradiated while being three-dimensionally scanned in such a way as to conform to the shape of a diseased site. The scanning irradiation method in which irradiation and non-irradiation of a pencil-shaped thin charged particle beam are alternatively repeated so that the pencil-shaped thin charged particle beam is irradiated in a spot shape and in a pointillism manner is referred to as the spot scanning method, in particular. The scanning irradiation method in which a pencil-shaped thin charged particle beam is scanned while being irradiated so that the pencil-shaped thin charged particle beam is irradiated in a one-stroke writing manner is referred to as the raster-scanning method, in particular. In each of the spot scanning method and the raster-scanning method, in order to scan a pencil-shaped thin charged particle beam, there is utilized an electromagnet, referred to as a scanning electromagnet or a “so-sa” electromagnet (referred to as a “scanning electromagnet”, hereinafter), that makes a magnetic field change at high speed.
It goes without saying that in order to acquire high scanning accuracy of a scanning electromagnet, i.e., high irradiation accuracy of a particle beam therapy system, it is required to appropriately control the scanning electromagnet. However, the irradiation accuracy of a particle beam therapy system is deteriorated with time, even though the particle beam therapy system is adjusted at an initial stage. Accordingly, it is desirable to display the irradiation position accuracy in such a way that the tendency thereof is observed at a glance, so that the irradiation accuracy of the particle beam therapy system can appropriately be maintained and the maintenance thereof can be performed. Patent Document 1 discloses a charged particle beam irradiation system in which based on a detection signal from a position monitor, the deviation (the difference from the desired value) of a beam irradiation position is calculated; it is determined whether or not the calculated deviation of a beam irradiation position has exceeded an allowable value; for example, in the case where it is determined that the deviation of a beam irradiation position has exceeded the allowable value, an interlock signal and a display signal are outputted to an output unit; and then the beam irradiation is stopped.